[AlexJei]

that's definitely a little high. despite what people tell you, the filters don't really have much of any effect on fuel economy. they might if they were REALLY trashed but that would take a lot. filters used to have an effect with carburetors but as soon as we started using fuel injection and sensors got better, all that happens is you lose some power in the higher RPM range.

worth knowing if you have manual or auto trans. tell us if there is anything unusual going on. noises? strange behavior? have you checked for ECU codes? any mods we should know about?

you're generally looking for anything that would waste engine power. a slipping belt, dragging brakes (the rear calipers are known to seize), fuel leak, misfires just to name a few. whats your mileage? was regular maintenance kept up? badly worn spark plugs could be a problem.

although very unlikely, the intake valves could be getting clogged up with carbon. i would leave that to last on the list to check. it's not really a problem on these cars but is not impossible. i'd guess usually with only driving short trips in the city would this ever be a problem and even then that would be rare.

give us all the information you can in as much detail as you can. fuel economy problems are difficult to sort out.

Actually a few days ago I noticed that the car is pulling to the left, it might be a seized break caliper but it also can be just a bad alignment, I will definitely check both of those as soon as possible.

For more details I can add that my Mazda came with automatic transmission. Before I bought this car I check it in local Mazda service, there were no ECU codes. Mechanics listened to the engine, they didn't hear anything strange neither, everything seemed fine. The car has 50 000 miles on it and as I said, I bought it a week ago so I didn't really maintained it yet, but last owner did every maintenance it needed, everything was checked and changed regularly.
P.S. It doesn't have any mods installed, everything is stock. It's just a car for everyday driving, which gets 15mpg in the city)).

 

[AlexJei]

You have only had the car a week...how are you determining your fuel consumption? Don't use the numbers on the dash display. Fill the tank and do the math.

Of course I'm not looking at the dash numbers, 5 days ago I filled the gas tank all the way up, drove the car for 100km(=62mi) and again filled it up until it was completely full. I did this to see how much fuel it consumes per 100km and the number was 16 litres.(=4.2gallons). Right now I'm trying another method. last time when I fully filled the car I also reseted a trip odometer. as you know Mazda 3 has 9 "Big Squares" and 6 "Small rectangles" on dashboard for fuel indication. I already drove 220km and Squares went down from 9 to 2. Mazda 3's tank size is 50L, each of "Big Squares" are equal to 5L. 5 x 7 = 35 | 35 / 220 = 0.159 | 0.159 x 100 = 15.9. and this is the math I'm frustrated with.

 

[AlexJei]

that would be a classic symptom of a seized caliper. if you put your hand near the wheels (don't touch!) after a 15 minute drive i'm going to guess one will feel hot.

if you find this is the case, here's some info. feel free to pass this on to your mechanic if you're not doing the work yourself. this is IF it is a rear caliper seizing.

- there's a recall for the parking brake. the cable that actuates the calipers has a rubber boot at the caliper end. the original boot can leak, make sure this recall has been done as it can cause a caliper to seize

- the rear brakes like to jam up anyway. root cause is corrosion buildup UNDER the pad shims. this pinches the pads and causes them to seize. this is not to say that other causes such as stuck slide pin or stuck piston shouldn't be investigated. fixing it is actually pretty easy. knock the pads out, you'll probably have to pound them with a hammer. before installing new pads/shims, use a heavy metal file (bastard file) on the channels where the shims go and the pad tabs ride. your goal should be to widen them a little. the corrosion has to be removed anyhow. next put a light coat of caliper grease there before putting the shims in. this will help prevent future corrosion and give it some room if it does start to form.

if it turns out not to be brakes, we can start exploring other possibilities. i'm not expecting much since you have no CEL. modern ECUs are very good and picking up small faults.

I'm definitely going to check if any of the wheels are getting hot and thanks for the info, it's really helpful.